The present invention relates to an image display medium, and more particularly, it relates to an image display medium using particles.
As an image display medium that can be repeatedly rewritten, a twisting ball display (utilizing rotatable particles coated with two colors), a medium utilizing electrophoresis or magnetophoresis, a thermal rewritable medium and a medium utilizing a liquid crystal that can store an image have been proposed. These image display media are excellent in storage stability of an image but have low contrast due to low whiteness of the background.
As an image display medium having a white background, a medium has been proposed, in which an electroconductive colored toner and white particles are filled between a pair of electrodes facing each other. In this image display medium, an electric charge is injected to the electroconductive colored toner through a charge transporting layer formed on the inner surface of the electrode of the non-display substrate, and the electroconductive colored toner having the electric charge injected migrates to the display substrate facing the non-display substrate by an electric field between the electrode substrates, whereby the electroconductive colored toner adheres on the inner surface of the display substrate to display an image (Theses from Japan Hardcopy, 1999, p. 249-252). This image display medium is formed only with solid materials, and white color display and black color display can be theoretically completely switched. In the image display medium, however, an electroconductive colored toner that is not in contact with the charge transporting layer provided on the inner surface of the electrode of the non-image display substrate and an electroconductive colored toner that is isolated from the other electroconductive colored toner are present, and these kinds of electroconductive colored toner do not migrate by the electric field but are present at random between the substrate because no electric charge is injected thereinto, and therefore improvement in contrast is insufficient.
The inventors have been proposed an image display medium containing a pair of substrates having filled therebetween plural kinds of particles having different colors and different charge characteristics (Japanese Patent Application No. 2000-165138). The image display medium can exhibit excellent contrast in the initial stage, but after subjecting rewriting for a long period of time, there are case where the image density is decreased to lower the contrast, and the image density becomes non-uniform to cause image unevenness.
The invention has been made in view of the foregoing circumstances and is to provide an image display medium that can display an image without image unevenness having high contrast even when the medium is repeatedly rewritten for a long period of time.
According to an aspect of the invention, the image display medium contains a first substrate transmitting light, a second substrate provided to face the first substrate, first and second particles having different colors filled between the first substrate and the second substrate, the first and second particles each contain mother particles and fine particles attached on a surface of the mother particles, the first particles are charged positively, and the second particles are charged negatively.
The first particles and the second particles collide with the particles, the substrates and a vessel due to agitation before filling and agitation in the initialization step after filling, and as a result, the first particles are charged positively, whereas the second particles are charged negatively. Thus, a Coulomb force is formed between the first particles and the second particles, and the first particles and the second particles intend to be attached and aggregated to each other. Upon application of an electric field to the substrates corresponding to an image signal, when an electrostatic force applied to the respective particles caused by the applied electric field is larger than the coulomb force between the first and second particles, and the image force and a contact potential force between the particles and the substrates, the first and second particles are separated from each other to migrate to the substrate having the polarity opposite to the polarity of the particles. The charged particles reaching the substrate are attached to and fixed on the substrate by an image force formed between the particles and the substrate and a van der Waals force between the particles and the substrate.
When the charge amounts of the respective particles are high, the aggregation force between the first and second particles is high, and thus the particles are difficult to be separated. Further, the particles having a large charge amount have a high adhesion property and are difficult to migrate even when an electric field is applied thereto, and the probability that the particles are fixed on the surface of the substrate becomes high. When aggregated particles having a high charge amount are separated, there is a possibility that electric discharge is locally caused, and the resulting charge amounts cannot be constant.
When the particles have a low charge amount, on the other hand, the respective particles are difficult to be affected by the external electric field, and the particles do not migrate but maintain a loosely aggregated state.
According to the invention, the first and second particles maintain suitable charge amounts for the respective particles owing to the presence of the fine particles on the surface of the mother particles. Therefore, the first and second particles are quickly separated by application of an electric field corresponding to an image signal to the substrates, and a desired image formed by contrast between the color of the first particles and the color of the second particles. The resulting image is maintained after turning off the electric field because the particles moving to the substrates stay thereon owing to the image force and a van der Waals force.
It is preferred that the mother particles of the first particles are those charged positively, and the mother particles of the second particles are those charged negatively. The fine particles of the first particles are preferably those charged positively, and specifically, fine particles treated with a silane coupling agent containing a nitrogen atom or a silicone oil containing a nitrogen atom are preferred. The fine particles of the second particles are preferably those charged negatively, and specifically, fine particles treated with a silane coupling agent containing atoms other than a nitrogen atom or a silicone oil containing atoms other than a nitrogen atom. The fine particles impart an appropriate charge amount, stable charge maintenance characteristics and a good fluidity to the particles, and they enable that the particles can repeatedly migrate between the substrates by the electric field applied between the substrates without firmly adhering on the inner surfaces of the substrates.
The first and/or second particles may further contain electroconductive fine particles. When the electroconductive fine particles that adequately cause exchange of the charge and leakage of the charge are present in combination with the fine particles, increase of the charge amount of the particles due to friction among the particles and friction between the particles and the substrates for a long period, i.e., so-called charge-up, can be avoided.
The fine particles are preferably firmly fixed on the surface of the mother particles by a heating treatment or a high speed airflow impact treatment. According to the treatment, even when collision among the particles and between the particles and the substrates repeatedly occurs due to the repeated migration of the particles to cause mechanical impact and frictional heat applied on the particles, the particles are prevented from detachment and flotage of the fine particles attached on the surface of the mother particles, attachment of the floating fine particles on the surface of the other particles, and burying on the surface of the particles, whereby fluctuation of the charge characteristics and the flowability in the initial stage is prevented.